1. Field of the Invention
This invention relates to epicyclic gear change assemblies which have two ratios, a direct drive or unit ratio and an overdrive or underdrive ratio. Such epicyclic gear change assemblies are commonly used as auxiliary ratio change means in motor vehicles and are normally, but not exclusively, arranged to give a direct drive ratio and an overdrive ratio.
The invention will be described in relation to an epicyclic gear change assembly capable of giving a direct drive ratio and an overdrive ratio but the invention is applicable to epicyclic gear change assemblies arranged to give direct and underdrive ratios and also to epicyclic gear change assemblies for purposes other than use as an auxiliary ratio change means in a motor vehicle but in which it is required to obtain a direct drive or unit ratio and a planetary ratio.
2. Description of the Prior Art
In presently known epicyclic gear change assemblies direct drive is normally obtained by the engagement of a friction clutch which runs in the lubricating oil of the assembly and which clutches together two members of the gearing. For overdrive ratio the clutch is disengaged and the reaction member of the gearing, normally the sun gear, is held stationary by means of a brake.
If such an epicyclic gear change assembly is being used as an auxiliary ratio change means in a motor vehicle having a main change speed gear box and if the assembly includes a one-way clutch between the input and output members of the assembly, or between the reaction member and the casing of the assembly, then the arrangement will be such that when reverse gear is selected in the main change speed gear box the epicyclic gear change assembly will be in direct drive. If the vehicle is brought to rest with the overdrive ratio engaged and then reverse is selected in the main change speed gear box, the epicyclic gear change assembly must move over to direct drive with the vehicle stationary. There is thus no relative rotation between the parts of the friction clutch which is engaged for direct drive and the oil in which the clutch runs will not be wiped off the clutch surfaces as occurs if the clutch is engaged while the parts thereof are rotating. Consequently, as soon as drive is commenced, the clutch will slip.
Normally, the clutch is engaged by a spring force but in some designs the engaging force of the spring is augmented by thrust generated by providing the gears of the gearing with teeth of a helical form. This thrust, however, is only generated when the clutch is engaged. If the clutch slips then full thrust is not generated and there is therefore a considerable drop in the actuating force for the clutch and there will be a tendency for the vehicle to roll forwards, if it is on a hill facing downwards and reverse has been selected, until such time as the clutch slip ceases.
It is an object of the present invention to provide an epicyclic gear change assembly for use as auxiliary ratio change means for a motor vehicle, in which a positive mechanical drive is established in direct drive if there is a tendency for the clutch to slip.